Devices have been proposed to combine the traditional refractive diagnostic instruments with optical coherence tomography and low-coherence interferometry devices. Examples are U.S. Pat. No. 6,799,891 by Barth and U.S. Publication No. 2005/0140981 by Waelti and U.S. Publication No. 2005/0203422 by Wei. Typically, the anterior (front) corneal shape is measured by imaging light reflected from the cornea, and the locations of deeper surfaces, back to the retina, are measured by low-coherence interferometry or some type of tomographic imaging.
Tomographic imaging techniques, such as Scheimpflug imaging, slit-lamp imaging, and optical coherence tomography (OCT), give a cross-sectional image of the refractive surfaces. These methods suffice for measuring the depth of the lens and thickness of the cornea to the required accuracy. Tomographic images alone, however, have not been proven accurate enough to determine refractive power of these surfaces; see for example, Tang et al (2006) who have recently compared the OCT-only approach, with combining OCT with anterior shape measurements from a separate device. The conversion from the measured profile, the “lens sag”, of the anterior surface of the cornea to refractive power is 1.3 μm/Diopter, if the sag is measured over a circular patch of cornea with 1 mm radius. Therefore measuring the refractive power to 0.25-Diopter accuracy requires sub-micron resolution of the corneal shape. The diffraction limit makes this resolution very difficult to achieve with non-contact visible light imaging.
Interferometric ranging was shown to locate the corneal surface within 2 μm (Huang et al. 1991) by finding the peak intensity from a specular reflection from the cornea; this publication proposed extending the technique to transverse scanning. A method of using interferometry to determine the shape of the anterior surface of the eye has been described by Hochberg and Baroth in U.S. Pat. No. 5,317,389. Hochberg and Baroth record the interferogram across a line on the cornea in one single snapshot, using bulk optics and an array detector instead of a scanner.
We see the need to determine the complete refractive diagnosis using a single non-contact measurement device. We describe here how to use a single optical coherence tomography (OCT) scanner alone to collect the required information with sufficient accuracy.